As more and more powerful lasers are adapted for practical uses such as surgery, it has become more and more important to ensure against accidental firing of the laser. This is particularly so in surgical laser applications where misfiring could cause serious damage to patients and medical personnel alike. Often, in surgical protocols lasers are only to be fired when the probe or handpiece or other part of the laser beam delivery system is in contact with the body part to be operated on. Detection of contact can be a difficult matter. In laser heart surgery, for example, electrical or even magnetic proximity detectors can interfere with the electric field of the heart and cause problems with the heart function and rhythm; mechanical sensors likewise can cause reflex reactions by the heart which can result in arrhythmia. Thermal detectors are ineffective, where, also as in heart surgery, body parts are chilled before surgery. Further, in thermal sensor devices it is possible that ambient heating could cause accidental firing of the laser.